This invention relates generally to systems for stimulating skeletal muscle, e.g., cardiac assist systems, and more particularly augmenting such systems to provide an ongoing determination of whether the skeletal muscle is being stimulated.
Cardiac assist systems are designed to aid patients with chronically and unacceptably low output who cannot have such cardiac output raised to an acceptable level by traditional treatments, e.g., pacing or drug therapy. A specific type of cardiac assist system to which this invention is addressed, as a preferred example, is cardiomyoplasty. Cardiomyoplasty is surgical procedure for treating chronic heart failure (CHF), whereby a patient""s latissimus dorsi (LD) muscle is elevated, dissected, passed into the thoracic cavity, and then wrapped and secured around the failing heart. The LD muscle is first gradually stimulated with electrical impulses for a period of time up to about twelve weeks, to convert the muscle to a fatigue-resistant state. Following this conditioning, the LD muscle is chronically stimulated to contract in synchrony with the heart, in order to provide hemodynamic assistance. During the chronic muscle stimulation, an implantable pulse generator senses contractions of the heart via one or more sensing leads and controls generation of stimulation of the appropriate nerves of the muscle tissue with burst signals adapted to cause the muscle tissue to contract in synchrony with the heart. As a result, the heart is assisted in its contractions, thereby raising the stroke volume, and thus the output.
Following transposition of the LD muscle into the thoracic cavity, there is currently no reliable method or device for determining if the muscle is in fact contracting when stimulated. Further, contractions may be weak or strong, and there is no reliable method of determining the strength of the contractions that do take place. In the prior art, implantable sensors have been proposed and tested, with varying degrees of as of yet unfulfilled promise. The most basic sensors provide a yes/no indication of muscle contraction, while more sophisticated sensors are designed to provide a quantitative indication of relative strength of the contraction or contractility. However, such more sophisticated sensors are bulky and generally not yet proven. There thus remains a need in the art for a method and subsystem for monitoring skeletal muscle contraction on a beat-by-beat basis, so as to provide information for controlling the output level of the pulses in each stimulation pulse burst.
As is known, a unique characteristic of cardiac muscle is that it is able to be paced and fully captured by the application of a single electrical impulse. Skeletal muscle, on the other hand, will yield only a twitch contraction to a single electrical impulse. To effect a full, tetanic contraction in skeletal muscle requires delivering a train of electrical pulses to that muscle. Electrical pulses are typically delivered via a pair of leads, e.g., intramuscular, epimysial, etc., or between one lead electrode placed within or on the muscle and the pulse generator case for unipolar stimulation. It has been noted that during stimulation of the skeletal muscle, when the muscle does contract, the lead electrodes move relative to each other. As the distance between the electrodes carried by the leads changes, so does the impedance between the leads. This invention takes advantage of the relationship between muscle length and inter-lead impedance, which is known to be a substantially linear relationship. Consequently, the basis of this invention is to measure the impedance between the stimulation electrodes during each pulse in a stimulation train, and from such impedance information determine whether a contraction has occurred, and the quality of the muscle contraction.
Although the preferred embodiment of this invention is set forth with the illustration of cardiomyoplasty, it is equally applicable to other procedures, including aorta myoplasty, incontinence treatment, and any other muscle-activated system.
There is provided a system and method for stimulating skeletal muscle as part of a therapeutic procedure, which includes determining when delivered stimulation has evoked contraction of the skeletal muscle. The system provides a controllable pulse generator for periodically generating a burst of stimulus pulses and leads for delivering the bursts to electrodes in the skeletal muscle, and impedance measuring circuitry for measuring the impedance between the electrodes at the time of each pulse of the series. The impedance measurements are processed, preferably including obtaining the derivative or change in impedance for each pulse burst, and the impedance data is analyzed to determine whether it reflects a full muscle contraction. In the event of determinations of no contraction, the system includes a feedback mechanism for adjusting the power output of the burst pulses, so as to provide cyclical stimulation sufficiently above threshold to provide reliable muscle contraction.